Motor Protection Start Inhibits

[rockman7892]

I'm trying to understand how to set up motor protection on a 5kV 300hp motor.

NEMA MG-1 states the following:

a. Two starts in succession, coasting to rest between starts, with the motor initially at ambient temperature.

b. One start with the motor initially at a temperature not exceeding its rated load operating temperature.

I'm trying to understand what these mean exactly. The way I understand this is that If you walk up to a motor that has been sitting cold (at ambient) then you are allowed two consecutive starts of this motor. If the motor is started then stopped right away for some reason then it can be started immediately as long as it comes to a rest between starts. What exactly does the "two starts in succession identify"

Consequently on the other hand, if you walk up to a motor that has been running for a while and therefore is above ambient tempearature and this motor is stopped then it cannot be started again for some time period, or can it?

What exactly do pars (a) and (b) mean? What is the difference between hot and cold starts /hr as I have also seen.

I'm trying learn (not actually doing) how start inhibit and restart times should be set up for a Multilin 369.

 

[itsmoked]

They mean exactly what you have 'guessed' they mean.

On a motor like that, pretty much if the motor has run in that last few hours it is one start per hour as it will take that motor many, many, hours to drop to ambient after having run.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

No, you have made one minor mistake in your interpretation of part b.

"b. One start with the motor initially at a temperature not exceeding its rated load operating temperature."

"Consequently on the other hand, if you walk up to a motor that has been running for a while and therefore is above ambient temperature and this motor is stopped then it cannot be started again for some time period, or can it?"

There is a difference between rated temperature and ambient temperature. The only time you would be restricted from re-starting a motor that has been running continuously is if the ambient was higher or greater than the motor rated operating temperature, which is unlikely.

So what part b means is, if the motor is not "cold", you can re-start, but only once without having to wait for the proscribed time interval. That time interval is important too, so pay attention to it. Not all of them are based on an hour, I have seen a large motor designed for one "warm" start per day!


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[rockman7892]

Ok let me see if I'm understanding this totally.

a. Two starts in succession, coasting to rest between starts, with the motor initially at ambient temperature.

This means as I said before that I can walk up to a cold motor start it, and if it trips or stops I can start it again after waiting 30s.
If it trips out or stops again after the second start how long must be waited before attempting a 3rd start?

b. One start with the motor initially at a temperature not exceeding its rated load operating temperature.

If a motor has been running for lets say 1hr and then stops I can immediately try to restart it after waiting 30s. However if it stops for some reason after this restart, then I have to wait the alloted time period of usually an hour. How do I find out exactly what this time period is? Is this what is meant by 3 cold starts / 2 hot starts per hour?

 

[waross]

I have used a timer to give this protection. If the motor is allowed one start per hour, a timer is started when the motor is started. The timer must time out befor the motor may be restarted. The timer continues timing after the motor stops.
If you are allowed one warm start you will have to add some logic to start the timer on the second start.
OR you can make some compromises and keep the circuit simple.
Another technique that works in some cases is to keep the motor running long enough that it is ready for an immediate restart.
We had a large compressor that was allowed 3 starts per hour.
It was the secondary compressor. It only came online when the primary compressor could not maintain adequate pressure. It may not be called for days and then, with heavy plant use of air, it may be called to run several times an hour. We arranged the controls so that when the motor started, it continued to run for 20 minutes, regardless of demand. That way it was always ready for a restart but never exceeded the 3 starts per hour limit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

b. One start with the motor initially at a temperature not exceeding its rated load operating temperature.

If the motor has be running at rated load or less than rated load then it CAN be restarted as soon as it comes to a rest.

If the motor is overloaded and then trips due to say an overload relay trip or a overcurrent relay trip then it CAN NOT not be restarted immediately.

It's not really this simple because the specification refers to the temperature of the motor and I'm using current/load in the example but I believe these writing these 2 conditions as above helps make it clear.

Try this for (hopefully) a more precise meaning:

The motor will reach a certain temperature when run at rated load. Call this temperature Tl. If the temperature of the motor is not above Tl when it has come to a stop then it can be immediately re-started. If the temperature of the motor is above Tl when it has come to a stop then it can not be re-started until the temperature of the motor drops below Tl.

 

[lansford]

One other aspect may be the limitation on the starting equipment. We had an MV motor on a Dragline MG set and the reduced voltage autotransformer was the limiting facility.
JIM

 

[rockman7892]

Lansford

This was my next question. This motor actually does have an Autotransformer which states the following:

1) Two starts from ambient not to exceed 30s each with a 30s rest in between.

2) One start for 30s only if motor controller was pre-loaded with the autotransformer off the circuit for at least an hour.

The way I interpret this is as follows: No start should ever exceed 30s. If I walk up to a cold motor and transformer I can start it twice consecutively waiting 30s inbetween. After two consecutive starts I have to wait some time period. I'm having trouble figuring out what this time period should be, or how much run time inbetween starts would still define consecutive starts?

The second part is where I'm getting confused. It references an hour timeframe but I'm not sure how this applies. The way I think it applies is that if the transformer was used for a start of the motor and the motor has been running, then 1hr must have ellapsed from when the motor had been started. I dont see the direct relationship however between the motor having been running and the Autotransformer. Why an hour time between starts if the motor has been running and no time if the motor was at ambient?

 

[rockman7892]

Based on the options I have avaliable with the Multilin it would be hard to set it to adhere to the Autotransformer rating.

I have the avaliable settings:

Single Shot Restart (only on overload trips)
Start Inhibit (looks at thermal capacitor of motor)
Max Starts/Hour Permissable
Time Btwn Starts Permissable
Restart Block

I thought I could set Max Starts/Hour to (1) to ensure that Autotransformer is offline for an hour while the motor is running as stated in the second part, but then I would not be able to acheive the two consecutive starts listed in the first part.

If I set the time between starts permissable to 30s and allow two starts per hour I can acheive the first part of the transformer rating but then would not be able to ensure that the transformer was offline for at least an hour with the motor running as stated in the second part.

Any suggestions?

 

[EEBubba]

Starting is very stressful on the motor. There are large magnetic forces and massive currents interacting inside the motor. Typically on larger motors the limiting factor on the number of consecutive starts is the rotor. On many smaller motors the limiting factor is the winding copper's ability to dissipate heat.

The rotor is abused by the relatively large current it is demanding to produce torque and the fact that there is no circulating air to cool it. This is exasperated by the fact that when the motor is at standstill the rotor will see 60 hertz power, once the motor is running the apparent frequency on in the rotor bars is much less (less than 1 hz).

All of this adds up to severe heating in the rotor bars and the shorting ring. Both copper bar and AL die cast rotors have these issues.

Auto transformer starting is a gentler method to start and the motor designer should be able to give an answer as to the maximum number of starts based on various timings of running, idle, and stops. I would send the application information and the requested starts and stops to the motor manufacturer, and get the answer from them.

The conditions of the starts and the runs are critical for understanding the thermal reactions of the rotor. Motors that are running either loaded or unloaded will have a lower rotor temperature than a "start-stop" situation, while running there is time and cooling air to allow the rotor to stabilize and exhaust temperature. The one hour running allows for the initial heating of the rotor to dissipate. It seems counterintuitive, but think of an over heating car, the last thing you want to do it turn it off and remove the only source of cooling the engine has... same principles here.

I wish i could help on the Multilin, but I am looking to learn a bit by reading others answers on that issue.

 

[rockman7892]

In researching this subject more I found the Attached GE document regarding "Cool Time Constant Calculations". I am thinking that following the approach listed in this document may be a way to acheive the necessary time delays for consecutive starts of a motor from ambient.

This document shows how to calculate the time constant (T) based upon motor information. Where on the motor datasheet however do you find what (t) should be for this equation. In the example shown they use 3hrs. I have attached the datasheet for the particular motor in question and was looking to see where I could find this (t) rating based off of the motor thermal curve?

Following this procedure should take care of letting my motor cool the appropriate amount of time after two consecutive starts from ambient. How can I follow this same procedure or something similar for allowing my motor to cool the necessary amount of time after it has been running? (I assume this time will be less b/c the cooling time constant of the motor will be less when running.)

 

[electricpete]

"(I assume this time will be less b/c the cooling time constant of the motor will be less when running.)"
Sounds like a correct assumption to me.

"I found the Attached GE document.."
I don't see any attachment


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[rockman7892]

I apoligize, I forgot to attach the document.

Attached is the GE document and motor datasheets that I was referring to.

 

[LionelHutz]

There is no number specifying the number of starts per hour or something similar that can be used to determine the cooling time in that datasheet.

Sometimes the datasheet will say something along the lines of "2 consecutive starts per hour" or "2 consecutive starts with the next start allowed in 90 minutes" or something like that. The cooling time can be inferred from those numbers.

Typically, you would use the same number for both running and stopped case unless the datasheet gave different cooling times or starts per hour for running vs stopped.

 

[rockman7892]

LionelHutz

Thanks for the response. What is typically done or assumed in a case like this where it does not specifically state on the motor datasheet the number of consecutive starts, or in a case like this where the datasheet does reference any starting times?

Can anything be inferred from the Motor Thermal Limit Curves associated with this datasheet?

 

[LionelHutz]

Well, you either have to ask the manufacturer again and get the data or make a guess, hopefully based on experience with similar motors.